1. Field of the Invention
This invention relates to a method of compensating for atmospheric distortion while using a radar to detect the position of an object that is near the horizon relative to the radar site and, more specifically, to a method of determining the temporary bending angle of the atmosphere by comparing the apparent position of low-elevation man-made satellites to the known position of the satellite.
2. Background Information
Radar waves, like all light waves, are subject to distortion as the waves pass through the atmosphere and, more specifically, the troposphere. The distortion of the waves is caused by both large-scale effects and small-scale effects. Large-scale effects include refraction due to layers of gas with different densities in the atmosphere. Small-scale effects are caused by turbulence and heat gradients. Refraction by large-scale effects cause an object outside the atmosphere to appear in a location other than the object""s actual location. Small-scale effects cause an object outside of the atmosphere to twinkle. The distortion due to large-scale refraction error for low elevation targets can be 2-10 mrad.
It is desirable to know the actual location of long range, low elevation objects such as, but not limited to, missiles as soon as possible so that an accurate trajectory may be determined. Thus, methods of correcting for the distortion caused by large scale effects have been developed. There are two common methods for determining the atmospheric xe2x80x9cbending angle,xe2x80x9d that is, the angle that the atmosphere bends light waves, caused by refraction. The Standard Atmospheric Model relies on a historical model of the atmosphere and on an assumption that earth and the atmosphere are spherical. This model is easy to compute and is functional for objects at higher elevations. However, the assumptions used to create this model result in a significant degree of error for objects at low elevations.
The second method relies on a three-dimensional model of the atmosphere based on National Weather Prediction data. The three-dimensional model is an improvement on the Standard Atmospheric Model as the assumptions are removed and replaced with data representative of the actual atmosphere. The National Weather Service collects data from satellites, balloons, ground stations and other sources and makes this data available to the public. The disadvantage to this method is that the National Weather Service data is only updated once about every six hours. Thus, the National Weather Prediction model relies on old data and may not be relevant in changing conditions.
There is, therefore, a need for a method of compensating for atmospheric effects while using radar to detect an object near the horizon that relies on near current data.
There is a further need for a method of compensating for atmospheric effects while using radar to detect an object near the horizon that can provide a basis for a refractivity model that incorporates local weather data.
There is a further need for a method of compensating for atmospheric effects while using radar to detect an object near the horizon that determines a real time bending angle by comparing the observed location of a low elevation object to the known location of the low elevation object.
There is a further need for a method of compensating for atmospheric effects that reduces the refraction error for objects at a low elevation to less than 50 grad.
There is a further need for a method of compensating for atmospheric effects that utilizes existing equipment.
These needs, and others, are met by the present invention which provides a method of compensating for atmospheric effects that determines the bending angle of the atmosphere about once every 30 minutes utilizing a plurality of man made satellites, such as the Global Positioning System (xe2x80x9cGPSxe2x80x9d) satellites. The GPS satellite system currently includes 26 satellites which rise or set about once every 30 minutes. It is desirable to use the GPS satellites in conjunction with this method as the locations of the GPS satellites are carefully monitored. Throughout the remainder of the specification, the name xe2x80x9cGPS satellitesxe2x80x9d will be used, however, any man-made satellite or satellite system may be used to perform the disclosed method.
Because the position of the GPS satellites is known, the bending angle of the atmosphere can be determined by comparing the apparent location of the satellite, that is the location as seen by the radar site and which is distorted due to atmospheric distortion, to the known location of the satellite. This calculation of the bending angle can be performed each time a GPS satellite rises or sets. Additionally, the calculation of the bending angle can be improved and adapted by recording relevant weather data at the radar site. The weather data may be collected in real time near the radar site, or may be acquired from the National Weather Service which provides global weather data. Such weather data can be used to create a three-dimensional model and used to further refine the calculation of the bending angle.
An alternative method may be performed using a second GPS receiver located a distance from the radar site. In this embodiment of the method, the first GPS receiver is stationed near the radar site, the second GPS receiver is a distance, preferably over 100 km, away from the radar site. The second receiver has a high elevation view of the GPS satellite when the radar site has a low elevation view of the GPS satellite. Thus, the position of the satellite as seen by the second receiver is not substantially affected by refraction. As such, the observed position of the satellite at the second receiver site may be used in place of the known location data. This method may be used if the data regarding the known location of the satellite is not available.
The alternative method may be improved upon by using data regarding the known location of the satellite. That is, when the GPS satellite is at a low elevation relative to the radar site, doppler data from the GPS satellite is collected by both the first and second receivers. The doppler frequency shift between the signal received by the two receivers is compared to the known position of the GPS satellite. This comparison allows removal of the phase changes due to clock drift and satellite motion. The remaining phase change is attributed to atmospheric effects. Again, local real time weather data or weather data from the National Weather Service is used to build a profile of the conditions in the lower atmosphere near the radar site.
Thus, the disclosed method is an improvement over the standard atmospheric model because the disclosed method does not rely on the assumption of spherical symmetry. The disclosed method is also an improvement over the three-dimensional refractivity model as the atmospheric data is updated about every 30 minutes. Moreover, because the position of the satellites is known, the accuracy of the bending angle calculation is such that when the calculated bending angle is applied to the apparent position of another object at a low elevation, the error in determining the object""s location is reduced to less than 50 xcexcrad.
It is an object of this invention to provide a method of compensating for atmospheric effects while using radar to detect an object near the horizon that relies on near current data
It is a further object of this invention to provide a method of compensating for atmospheric effects while using radar to detect an object near the horizon that can provide a basis for a refractivity model that incorporates local weather data.
It is a further object of this invention to provide a method of compensating for atmospheric effects while using radar to detect an object near the horizon that determines a real time bending angle by comparing the observed location of a low elevation object to the known location of the low elevation object.
It is a further object of this invention to provide a method of compensating for atmospheric effects that reduces the refraction error at low elevations to less than 50 xcexcrad.
It is a further object of this invention to provide a method of compensating for atmospheric effects that utilizes existing equipment.